Puzzles, facts, lessons from dileptons in few GeV energy regime"

Transcription

1 International Workshop on In-Medium Effects in Hadronic and Partonic Systems Obergurgl, Austria, February 1-5, 011" Puzzles, facts, lessons from dileptons in few GeV energy regime" Tetyana Galatyuk for the HADES Collaboration" Goethe-Universität Frankfurt / EMMI"

2 Nuclear matter in the universe" From the Big Bang to the galaxies ρ 0 = 0.16 fm -3" (ρ)

3 Nuclear matter in the universe" From the Big Bang to the galaxies Accessible through heavy-ion collisions " ρ 0 = 0.16 fm -3" (ρ)

4 Transient existence of a dense/hot phase" first-chance collisions! Challenge! dense phase:! τ < fm/c!!extract information on the high density phase! freeze-out!

5 Transient existence of a dense/hot phase" N times" first-chance collisions! Challenge! dense phase:! τ < fm/c!!extract information on the high density phase! Is extreme matter created at SIS?! freeze-out! Same in the whole volume or <dense and corona like>!

6 The SIS heavy-ion energy regime baryon dominated!" Evolution of average ρ B ( τ system ) ρ Β / ρ 0 ρ (ρ 0 ) τ >15 fm/c" Composition of a hot πδn gas (T) Long lifetime of the high density phase:! System stays above ground state density for > 15 fm/c " densities: ρ max /ρ 0 = -3! temperature: T < 0 MeV! Baryon dominated:" o π densities a factor ~0 lower as compared to SPS regime!! o matter dominated by baryonic resonances (~30%), N π /A part %! S. Vogel et al., Phys. Rev. C 78 (008) R. Rapp and J. Wambach, ArXiv:hep-ph/99099v1 I prefer penetrating probes

7 Electromagnetic structure of dense/hot matter" γ * l + l - Bremsstrahlung! l - l + γ * Resonance decay! Lepton pairs couple to hadrons through time-like virtual photons.! The reconstruction of virtual photons via dileptons gives access to the electromagnetic properties of nuclear matter under extreme conditions.! Observable: short-lived vector mesons (ρ, ω, φ)! Strong coupling of γ* to Vector Meson VMD model! l + γ * Decays of (long-lived) neutral mesons (π 0, η, ω)! l -

8 The ρ meson in nuclear matter" ρ π Δ N -1 N* > > N -1 ρ Additional contributions to the ρ-meson self-energy in the medium" Dalitz decay of resonances" Meson nucleon molecules?" S. Leupold, U. Mosel, Post et al." NPA 741 (004) 81, NPA 780 (006) 187" ρ/ρ 0 = "

9 The experimental challenge..." Thereʼs no such thing as a free lunch +" Lepton pairs are rare probes (Branching Ratio < -4 )! at SIS energies sub-threshold Vector Meson production! Large combinatorial background in e + e - from:! Dalitz decays (π 0 )! Conversion pairs! + H.Wallace, 1943"

10 The DLS at LBNL " The puzzle!! Observed excess remained unexplained over years Calculation: E.L.Bratkovskaya et al.! Phys. Lett. B445 (1999) 65" ρ N*(150)! all! Calculation: Ernst et al.! Phys. Rev. C58 (1998) 447" Calculation: C. Fuchs et al.! Phys. Rev. C68 (003) " 1 AGeV

11 Even more puzzling" No consistent explanation of pair production in elementary reactions" UrQMD C. Ernst et al. PRC 58 (1997) 447

12 High Acceptance Di-Electron Spectrometer" HADES strategy:! Systematic di-electron spectroscopy in NN, AA, pa, πn and πa collisions" o Beams provided by SIS18: π, p, nuclei! o Full azimuthal coverage! o Hadron and lepton PID! o e + e - pair acceptance 0.35! o Mass resolution % (ρ/ω region)! o ~ channels! o now: 0 khz event rate (500 Mbyte/s peak data rate)! FW

13 Is there any medium radiation?" e + e - η π 0 Efficiency corrected di-electron spectra normalized to the number of neutral pions! e + first-chance collisions! First chance NN collisions! Baryonic sources:! NN bremsstrahlung! Resonances (Δ, N * ) Ne + e - Freeze-out: long lived mesons! dense phase! π 0 / η - fixed by TAPS data" freeze-out! R. Averbeck et al., 1997 Z. Phys. A " Enhanced pair yield above η-contribution established!" Phys. Lett. B 663 (008) 43"

14 Is there any medium radiation?" e + e - η π 0 Efficiency corrected di-electron spectra normalized to the number of neutral pions! e + first-chance collisions! First chance NN collisions! Baryonic sources:! NN bremsstrahlung! Resonances (Δ, N * ) Ne + e - Freeze-out: long lived mesons! dense phase! π 0 / η - fixed by TAPS data" freeze-out! R. Averbeck et al., 1997 Z. Phys. A " Enhanced pair yield above η-contribution established!" True excess from dense phase?! Phys. Lett. B 663 (008) 43" Contribution from the initial phase?!

15 p. 480" At Berkeley BEVALAC the DLS spectrometer was built to study dileptons... A striking dilepton signal was found, but my guess is its reality will probably never be understood, although this mass/energy region will be soon tested by the HADES experiment at GSI " E. Shuryak "

16 DLS puzzle solved by Bremsstrahlung?" HADES data in the acceptance of DLS, compared to DLS data" 1997: Bremsstrahlung contribution was found to be small! 007: DLS puzzle may be solved when incorporating a stronger Bremsstrahlung contribution???!

17 Elementary reactions with HADES" If you are out to describe the truth, leave elegance to the tailor +" + A. Einstein" Goal:" Understand baryonic lepton pair production! Establish cocktail of free hadron decays reference for HI collisions!

18 Elementary reactions with HADES" If you are out to describe the truth, leave elegance to the tailor +" + A. Einstein" Goal:" ] [!b/gev/c d/dm ee Understand baryonic lepton pair production! Establish cocktail of free hadron decays reference for HI collisions! p + p 1.5 GeV o e - > 9 + e [GeV/c ] M ee ] [!b/gev/c d/dm ee quasi-free n + p 1.5 GeV o e - > 9 + e [GeV/c ] M ee

19 Origen of the low-mass pair excess in C+C collisions " e + Definition of the Reference data! γ * e - )] [1/(GeV/c dn corr /dm ee 0 1/N comp. subtracted 1 e + - e 1 > 9 o N+N ee / dn/dm A+A ee dn/dm C+ C AGeV 1 1 C+ C 1 AGeV 1 (pp+np) 1.5 GeV [GeV/c HADES: Phys. Lett. B 690 (0) M ee M ee ] [GeV/c ] Lesson: know your reference! N N N N N N Δ, N *! Δ, N *! γ * e + N N N N N e - C. Fuchs, PRC 68, (003)

20 HADES pp and dp (tagged n) data vs. models" ] [!b/gev/c d/dm ee p + p 1.5 GeV o e + - > 9 e 0 OBE KK OBE SM M ee [GeV/c ] Phys. Lett. B 690 (0) 118 ] [!b/gev/c d/dm ee quasi-free n + p 1.5 GeV o e + - > 9 e 0 + OBE KK+ OBE SM [GeV/c ] N N q = M inv (e+ e - ) = M γ* > 0 M ee Δ γ * e + N N e -

21 Close to a theoretical explanation" One Boson Exchange effective Lagrangian based approach including pion electromagnetic form factor! N N N *, Δ ρ R. Shyam and U. Mosel, Phys. Rev. C 8:0601, 0" γ N N

22 Elementary reactions in transport models" )] [1/(GeV/c dn corr /dm ee 1/N p + p 1.5 GeV o e - > 9 + e [GeV/c ] 0 NN Bremsst. total M ee HSD )] [1/(GeV/c dn corr /dm ee 1/N NN bremsstrahlung are calculated within the SPA model, restricting the emission process to elastic NN collisions! d + p 1.5 GeV o e - > 9 + e 0 NN Bremsst. N Bremsst. total [GeV/c ] Δ contribution is treated explicitly by producing and decaying the resonance within a Dalitz-decay model in inelastic collisions! The interference of elastic and inelastic channels is neglected.! M ee HSD

23 The interference effects" M inv distribution of e + e in pp and pn collisions as a coherent sum (solid lines) vs. an incoherent summation (dashed lines)! L. Kaptari et al. arxiv: v1 [nucl-th]" The interference effects become significant at higher values of the di-electron invariant mass and reduce the cross section by a factor of about.5!"

24 Investigation of ω meson modifications at nuclear ground state density" ] [µb/gev/c d!/dm ee Efficiency corrected data, normalized to the number of pp elastic events ] [µb/gev/c d!/dm ee " M ee [GeV/c ] p + p 3.5 GeV o " e - > 9 + e [GeV/c ] Results:" M ee Mass resolution: σ ω = 16 MeV/c! VM region practically background free: S/B ratio >! Efficiency corrected data, normalized to π 0 yield (via measured π ± )

25 Electron pairs from pp collisions at 3.5 GeV " Extraction of inclusive cross sections by fitting conventional sources to the experimental spectrum (PLUTO event generator):" Comparison with Pluto cocktail! π o :!15 mb! Δ:!fixed to π o! η:!1.04 mb! ω:!0.8 mb! ρ:!0.36 mb! The PDG 0 value for η e + e - BR has to be scaled down by a factor of at least 3."

26 Electron pairs from pp collisions at 3.5 GeV " Δ-N transition EM form factor Two component VDM type model only ρ is relevant in case of Δ reproduces simultaneously nucleon space-like and time-like as well as N-Δ space like form factors.! measured! space like" q < 0" unknown! time like" q > 0" Comparison with Pluto cocktail!

27 In-medium (cold nuclear matter) effects" Laboratory momentum distributions for inside and outside decay of ω Fast and slow e + e - sources! 35% of pairs with p < 800 MeV/c in ρ/ω mass range! fast e + e -! slow e + e -!

28 Is there physics beyond free N+N? " Efficiency corrected dielectron spectra from Ar+KCl at E kin = 1.76 GeV/u First observation of ωe + e - peak in heavy-ion collisions at SIS energies First evidence for true excess (strong excess above the reference cocktail )!½[pp+pn]=C+C! x.5-3! HADES: Nuclear Physics, A 830 (009) ω e + e -! Excess yield scales with system " size ~ A part 1.4 ( multi step processes)

29 Is there physics beyond free N+N? " Efficiency corrected dielectron spectra from Ar+KCl at E kin = 1.76 GeV/u First observation of ωe + e - peak in heavy-ion collisions at SIS energies First evidence for true excess (strong excess above the reference cocktail )!½[pp+pn]=C+C! x.5-3! HADES: Nuclear Physics, A 830 (009) ω e + e -! Excess yield scales with system " size ~ A part 1.4 ( multi step processes) N" N" N"... π R R N" π N" N" N" R γ * e + N" e - τ coll. 1 fm/c (00 MeV)"

30 Dilepton pair excess: from SPS to SIS... " CERES: Phys. Rev. Lett. 91 (003) 04301" 1 ) > (0 MeV/c ee >/<N ch 1 ) > (0 MeV/c ch ρ 6 7 PbAu 40 AGeV / geo 30 % PbAu 40 AGeV <dn ch /d>=16 30 % ee ee ee π Δ N -1 ee 0.11< <.64 p t >0. GeV/c ee >35 mrad N* > > N -1 o Main source: π + π ρ e + e - ee / geo <dn ch /d>=16.11< <.64 p t >0. GeV/c ee >35 mrad o Strength of dilepton yield at low masses is due to coupling to baryons! ρ e

31 Dilepton pair excess: from SPS to SIS... " CERES: Phys. Rev. Lett. 91 (003) 04301" 1 ) > (0 MeV/c ee >/<N ch ) > (0 MeV/c 5 5 ch 6 ρ 6 PbAu 40 AGeV / geo 30 % PbAu 40 AGeV <dn ch /d>=16 30 % ee π Δ N -1.11< <.64 p t >0. GeV/c ee >35 mrad N* > > N -1 o Main source: π + π ρ e + e - o Strength of dilepton yield at low masses is due to coupling to baryons! 7 ee ee ee 0 ee / geo <dn ch /d>=16.11< <.64 p t >0. GeV/c ee >35 mrad ρ e

32 Dilepton pair excess: from SPS to SIS... " CERES: Phys. Rev. Lett. 91 (003) 04301" e + e - pair yield Ar+KCl 1.76 GeV/u! 1 ) > (0 MeV/c ee >/<N ch 1 ) > (0 MeV/c ch ρ 6 7 PbAu 40 AGeV / geo 30 % PbAu 40 AGeV <dn ch /d>=16 30 % ee ee ee π Δ N -1 ee 0.11< <.64 p t >0. GeV/c ee >35 mrad N* > > N -1 o Main source: π + π ρ e + e - ee / geo <dn ch /d>=16.11< <.64 p t >0. GeV/c ee >35 mrad o Strength of dilepton yield at low masses is due to coupling to baryons! ρ e N * N ρ γ * l + o Dalitz decays of baryonic resonances - dominant source at low beam energies. l -

33 Hadron multiplicities at SIS and Statistical Hadronization Model" THERMUS calculation: T, µ b and R C fit to HADES Ar+KCl (1.76 GeV/u) data Extracted (T, µ b ) fit in the systematics of the Statistical Hadronization Model THERMUS fit: S.Wheaton and J.Cleymans," J.Phys.G31(005)S69" Thermal equilibration also at low energies (high µ B ) in particular concerning multi-strange hadrons?! Statistical Hadronization Model describes hadron abundances accept in case of:! large Ξ - (s=-) yield!

34 Strangeness production in 1.76 GeV/u Ar+KCl" Ξ - Λ+π - o Double strange baryon o First observation at SIS s- s th = -630MeV Extreme and exotic phase of Nuclear Matter at SIS q q q q q q q q q q q s q q q q q q s q Phys. Rev. Let., 3, 13301, (009)" q q q Strange quarks trapped in bubbles?!

35 Searching for landmarks of the phase diagram of matter at high µ B" ALICE, STAR, PHENIX! CERES, NA60! Highly interesting results from experiments importance of baryons! Quark-Gluon Plasma" Cross over" Critical point? (Lattice QCD)" Hadron gas" 1st order phase boundary?" Chiral? Quarkyonic? Nuclei DLS, HADES!

36 Searching for landmarks of the phase diagram of matter at high µ B" ALICE, STAR, PHENIX! CERES, NA60! Highly interesting results from experiments importance of baryons! Quark-Gluon Plasma" Cross over" Critical point? (Lattice QCD)" Hadron gas" 1st order phase boundary?" Chiral? Quarkyonic? Nuclei No measurements for beam energies of 40 GeV/u (high baryon densities)! Search (in this region) for exotic phases! DLS, HADES!

37 HADES upgrade: The RPC time-of-flight" RPC" K - acceptance (nice y CM coverage!) RPC" Full-system test results: " σ t 78 ps" σ x 8 mm" ε ~ 97%" Leading institute:! Coimbra, Portugal! σ t (ps) σ x (mm) Timing resolution" D. Belver et al. NIM A60(008) 687, 788! E. Blanco et al. NIM A60(008) 691! Row! Spatial resolution"

38 HADES DAQ upgrade" TRB +! MDC AddOn! TRBnet! to the front end electronics TRB +! TOF AddOn! TRB + Shower AddOn! TRB +! RICH AddOn! TRB for RPC! TRB! for Time Wall,Start,Veto! +... Ethernet! Parallel Event Building (computers)! LVL1 Trigger box! Commissioning run (Au+Au) November 0" 0kHz Event rate and 50 Mbyte/s Data rate!

39 Summary" HADES provides high-quality data for understanding di-electron and strangeness production in elementary and heavy-ion collisions at SIS energy regime." Long-lived states of compressed nuclear matter are produced in HIC at few GeV energy regime This state of matter might be much more exotic" as a hadron gas Observations: o o Thermal hadron production (with some exceptions) Strong broadening of in-medium states

40 The future at FAIR" SIS0:" Joint running of HADES and precbm:! Emissivity of hot/dense nuclear matter! Spectral functions of ρ/ω in dense (baryon dominated) hadronic matter! Multi-strange particle and lepton pair excitation functions! Charm production in proton induced reactions! SIS300:" Full exploitation of rare probes a highest µ B ; fluctuations, flow! CBM 8 45 AGeV HADES 8 AGeV

41 The HADES Collaboration" Cyprus:! Department of Physics, University of Cyprus! Czech Republic:! Nuclear Physics Institute, Academy of Sciences of Czech Republic! France:! IPN (UMR 8608), Université Paris Sud! Germany:! GSI, Darmstadt! FZ Dresden-Rossendorf! IKF, Goethe-Universität Frankfurt! II.PI, Justus Liebig Universität Giessen! PD E1, Technische Universität München! Italy:! Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali del Sud! Istituto Nazionale di Fisica Nucleare, Sezione di Milano! Poland:! Smoluchowski Institute of Physics, Jagiellonian University of Cracow! Portugal:! LIP-Laboratório de Instrumentação e Física Experimental de Partículas! Russia:! INR, Russian Academy of Science! Joint Institute of Nuclear Research! ITEP! Spain:! Departamento de Física de Partículas, University of Santiago de Compostela! Instituto de Física Corpuscular, Universidad de Valencia-CSIC! 17 institutions > 150 members"

42 Thanks for the dilepton workshop in 1991 " HADES" Father"

43 Bonus slides

44 Exclusive Δ pe + e - in p+p collisions at E kin = 1.5 GeV" Do we really measure pppδ + ppe + e -?! " "" "Check by detecting three particles in " " "the final state, i.e. p, e +, e - Mass of missing particle (M miss )! M ee >140 MeV/c " Identification of the final state vs. less acceptance! Much lower background! Allows determination of the:! Δ pe + e - branching ratio! Electromagnetic transition form factor?! Invariant mass distribution of pe + e -! 00 events" for M e+e- > 140 MeV/c " pp pδ + ppe + e -

45 Exclusive Δ pe + e - : angular distributions" pp data at high e + e - masses is dominated by Δ Dalitz decay! cm. angular distribution of pe + e - data points" simulated Δ Dalitz " First measurement of Δ Dalitz decays branching ratio in agreement within 0% with QED value of 4. -5! Helicity angle: angle between a lepton in the γ* rest frame, and a γ* in the Δ rest frame." consistent with OPE calcula7ons Helicity angle! data points" fitted with 1+b*cos (θ)" b = 1.1±0.3 consistent with QED calcula7ons

46 Systematic of the excess yield" Data for π 0 and η from TAPS collaboration! Dilepton excess scales with beam energy like π production! C+C (DLS)! C+C (HADES)! Ca+Ca (DLS)! Ar+KCl (HADES)! Excess yield scales stronger than linear with A part" 46!